Chemical Change and Energy 321 



FIG. 169. Regulating the air opening in a gas stove. 



" Water makes things decay more quickly ; there must 

 have been a drought and the trees must have fallen on dry 

 ground," said the second. 



Which was right ? 



Application 72. A gas stove had a yellow flame. In 

 front, by the handles, was a metal disk with holes so arranged 

 that turning it to the left allowed air to mix with the gas on 

 the way to the flame, and turning it to the right shut the air 

 off (see Fig. 170). 



One member of the family said, " Turn the disk to the 

 left and let more air mix with the gas." 



But another objected. "It has too much air already; 

 that's why the flame is yellow. Turn it to the right and 

 shut off the air from below." 



" You're both wrong. Why do you want to change it ? " 

 said a third member of the family. " The yellow flame is 

 the hottest, anyway. Can't you see that the yellow flame 

 gives more light? And don't you know that light is just a 



